1. Field of the Invention
This invention relates to the field of digital data processing and more particularly to time domain digital processing for use in sonar applications.
2. Description of Related Art
One of the greatest threats posed to the safety of a navy's surface ships is still that of the submarine and other underwater weapons. To this end sonar systems have become increasingly complex in order to enable the detection of such submarines or underwater weapons which themselves have been increasingly designed to present a very low observable return to such sonar systems.
In order to counter the increasingly lower observable returns modern sonar systems have become far more complex such that significant increases in the amount of data which is required to be processed are now required. Modern data storage and processing techniques have been applied to such sonar systems with the net result being a substantial increase in the volume of both analogue and digital data which is required to be processed in order to detect such underwater weapons.
The process by which a modern sonar system is able to look in a particular direction in order to locate objects under water is known as beamforming. Conventional beamforming techniques used with state of the art sonar systems require very high levels of performance from digital signal processing hardware. Such sonar systems utilise complex or large arrays of underwater hydrophones in order to receive the sonar information and pass it to the signal processing hardware.
Due to the high levels of data which are now required to be processed, conventional methods of digital signal processing are beginning to reach the upper limits of capability in both data management and storage.
Within the field of digital signal processing two differing techniques are used for managing the input data, namely frequency domain and time domain processing.
In simpler sonar systems where time domain techniques are adopted methods such as up-sampling or over sampling are used to analyse and manipulate data. Such a system requires only simple "shift and add" digital signal processing and as such a direct wide band of operation can be used.
When the levels of data or coefficients required to be processed in sonar systems reaches the level where the storage and signal processing requirements of time domain processing render the system inefficient or slow, frequency domain techniques are used. Such techniques offer some advantages in signal processing efficiency and data management due to the fact that they do not need to up sample or over sample incoming sensor data. Difficulties however occur in frequency domain processing due to the fact that only a narrow band of operation may be analysed at any one time and the process of splitting out frequency and phase information from such incoming sensor data. Frequency domain processing systems can operate a phase shift process in time for only one frequency, whereas in time domain systems no errors occur with frequency changes.
Research into this field has indicated that time domain approaches have the potential to offer the best signal performance tempered with a high processing load requirement whereas frequency domain techniques offer the best processing performance with a minimum of hardware. Research has also indicated that small systems (especially if a wide band of operation is required) time domain techniques offer the simplest and most straight forward and timely solution providing the proposed hardware is sufficiently capable.
Conventional beamforming techniques within modern sonar's require very high levels of performance from Digital Signal Processing (DSP) hardware. Complex or large sonar arrays often adopt frequency domain techniques because they offer advantages of signal processing efficiency and data management in not needing to upsample incoming sensor data. In simpler systems where upsampling or over-sampling can be tolerated, time domain processing is preferred because algorithms require only simple "shift & add" DSP and offer direct wide band operation.